Dual-diversity reception systems feature a pair of receivers configured to provide both diversity reception functionality, and redundancy such that should one of the receivers fail, the remaining receiver can take over thereby avoiding any loss of service. Such dual-diversity arrangements are common in wideband receivers.
There is a common relatively low frequency reference oscillator used by each receiver to drive the frequency of a respective much higher frequency local oscillator through a phase locked loop. Two independent higher frequency local oscillators are required for redundancy.
Diversity outputs are often combined using maximum ratio combining. It is important that the relative phase of the diversity outputs be somewhat constant within a burst for this maximum ratio combining to be effective. Using conventional phase locked loops avoids the requirement for complex custom multiloop phase locked loops. However, these phase locked loops utilize dividers with large local oscillator-to-reference frequency ratios, for example as large as 50000. This means that the high frequency oscillators in the two receivers are updated relatively infrequently, for example every 0.2 .mu.s. The relative phases of the two local oscillators are thus allowed to drift relative to one another. The result is considerable phase jitter between the two oscillators' output frequencies even though they are derived from the same reference.
While it would be possible to have both receivers configured to exist on the same circuit board using a common oscillator, this defeats one of the advantages of having a diversity reception system, namely that protection is provided against single point failure. If one receiver fails, the remaining of the two receivers can continue to be used with some reduction in sensitivity. The fact that the two receivers are implemented on separate circuit boards further complicates the task of getting the oscillators to operate in phase.